Rim diameter (15 in 215/65R15)
The last number in a tire size, such as 15 in P215/65R15, is the rim diameter: the diameter in inches of the wheel the tire fits.
How to read it
NHTSA’s booklet describes it as the wheel or rim diameter in inches, and Yokohama as the size of the wheel the tire is intended to fit. It is given in inches even in metric sizes, where the width is in millimetres: Michelin reads 205/55R17 as 205 mm wide and 17 inches in wheel diameter. FMVSS 110 defines rim diameter as the nominal diameter of the bead seat, the part of the wheel the tire’s beads sit on.
Also searched as: last number on tire, wheel size.
What each marking means under the standard that defines it. This page describes markings; it doesn’t advise on choosing, fitting, inflating or replacing tires.
Tire and wheel
FMVSS 139 requires each tire to fit each rim listed for its size designation. The listings come from the tire’s maker or from the published standards of the Tire and Rim Association, ETRTO and the Japanese tire makers’ association.
The size marked on the wheel
A wheel carries its own size, which gives the rim’s width as well as its diameter. FMVSS 110 defines the rim size designation as the rim diameter and width, the width being the distance between the rim flanges, and gives “20 × 5.50” as an example of how it is marked.
More tire sidewall markings
Browse symbols drawn with curves and waves or letters and numbers.
Sources
- Tire Safety: Everything Rides On It, DOT HS 809 361, U.S. National Highway Traffic Safety Administration (GovInfo), Rim Diameter Code
- Tire selection and rims and motor home/recreation vehicle trailer load carrying capacity information for motor vehicles with a GVWR of 4,536 kilograms (10,000 pounds) or less, 49 CFR 571.110 (FMVSS 110), U.S. National Highway Traffic Safety Administration (Electronic Code of Federal Regulations), S3, definitions; S5.2(b)
- How to Read Tire Markings and Sidewall Codes, Michelin North America, How to Read Tire Width and Aspect Ratio; Construction Type and Wheel Diameter
- Sizing Information, Yokohama Tire Corporation, Sizing Information
- Standard No. 139: New pneumatic radial tires for light vehicles, 49 CFR 571.139 (FMVSS 139), U.S. National Highway Traffic Safety Administration (Electronic Code of Federal Regulations), S4.1.1(a), (b); S5.1
- Frequently Asked Questions, Carlisle Transportation Products (The Carlstar Group), FAQ
Last reviewed September 29, 2026 by Rory Hansen. How we research